Deprecating backup(save), restore(open)

[fumitoh]

Backup and restore methods/functions (and their aliases save/open) will be deprecated and will be removed in near future.
These methods use the pickle/unpickle mechanism from Python's standard library, and it's getting hard to maintain without breaking the integrity of modelx's internals due to pickle/unpickle's unintended behavior.
There exist read/write methods for saving models and with the zip method you can save a model in one file, so It's not worth keeping backup/restore.